Those display panel or device substrates (in particular of the active matrix type) for use in liquid crystal display devices and organic EL display devices, color filter substrates, solar cell substrates and the like which are made of glass are generally in wide use. In recent years, however, for the reasons that glass sheets are breakable, inflexible, high in specific gravity and unsuited to weight reduction, various plastics materials have been investigated as substitutes therefor. For example, Laid-open Japanese Patent Application (JP Kokai) H06-337408 and JP Kokai H07-120740 describe transparent resin substrates for liquid crystal display devices as made of curing products obtained by curing an epoxy resin composition comprising an epoxy resin, an acid anhydride type curing agent and a curing catalyst. However, these prior art plastics materials as substitutes for glass are high in coefficient of linear expansion as compared with glass sheets and, when used as active matrix display device substrates, in particular, they may cause such problems as warping and aluminum wiring breaking in the production process and, accordingly, it is difficult to apply them to such fields of use. Thus, plastics materials satisfying the transparency, solvent resistance, liquid crystal resistance and heat resistance requirements, among others, imposed on display device substrates, in particular active matrix display device substrates and, at the same time, showing a small coefficient of linear expansion are demanded.
For reducing the coefficient of linear expansion, attempts have been made in the art to produce composite materials by incorporating an inorganic filler, such as a glass powder or glass fiber, in resins. In the case of such resin-inorganic filler composite materials, however, the transparency of the substrates is often impaired. The main cause is the difference in refractive index between the inorganic filler and resin, which causes diffused refraction of the light passing through the resin.
To solve such problems, various investigations have already been made to attain transparency by matching the refractive index of the resin with that of the inorganic filler. Various epoxy resin compositions rendered transparent by refractive index matching for use in photosemiconductor devices have been reported. For example, JP Kokai H04-236217 discloses a light-transmitting epoxy resin composition comprising an acid anhydride-cured epoxy resin and a filler substantially identical in refractive index to the resin. However, the fillers used in the prior art photosemiconductor device materials are special ones given a controlled refractive index by adjusting the silica-titania composition. Accordingly, it is demanded that technologies be developed for providing such resin compositions usable in a wider range of application fields by utilizing general-purpose glass fillers.